1. Field of the Invention
The present invention relates to the structure of a mold for press molding, and more particularly to such structure capable of maintaining a satisfactory precision in thickness of the molded article and also maintaining a long service life of the mold. The present invention is advantageously applicable, for example, to a mold for rapidly producing an optical element with optically functional surface from a molding material by pressing.
2. Related Background Art
There has recently been developed a method of producing an optical element provided with an optically functional face, by placing a molding material, such as a glass blank pre-molded to the shape and surface precision of a certain extent, in a mold of a desired surface precision and press molding said material under heating, thereby dispensing with post-finishing such as grinding and polishing.
Said press molding method consists of the steps of: positioning at least a pair of mold members so as to be capable of reciprocating in an opposite direction, relatively introducing a molding material into a cavity formed by members including said paired mold members; establishing a non-oxidative atmosphere, for example, nitrogen atmosphere for preventing the oxidation of the mold member; heating the mold members to a temperature enabling the molding, for example, a temperature where the viscosity of the molding material becomes to 10.sup.8 to 10.sup.12 poise; closing the mold members and effecting the pressing for a suitable period thereby transferring the surface form of the mold members to the molding material; then cooling the mold members to a temperature sufficiently lower than the glass transition temperature of the molding material; and removing the pressing pressure, to take out the molded optical element by opening the mold members.
The press molding method explained above enables to produce, for example, aspherical lens which has been extremely expensive, with a relatively low cost.
In the mold members for the press molding explained above, the thickness of the molded article, namely the thickness in a direction along which said paired mold-members are mutually opposed, is determined by stop position where said paired mold members are stopped to move in the mutual opposed directions upon pressing operation. Conventionally, a sleeve-shaped cylindrical mold member is employed for setting said stop position, and the thickness of the molded article is determined by impingement of the paired mold members directly or indirectly to said cylindrical mold member, or by fixing one of the paired mold members with said cylindrical mold member and impinging the other on said cylindrical mold member.
Such structure is disclosed for example in the Japanese Patent Laid-open Application No. 61-174127 and the Japanese Utility Model Laid-open Application No. 62-203235.
In such conventional structure, however, the impingement between the cylindrical mold member and at least one of the paired mold members takes place over an entire end face of the cylindrical mold member, and eventual presence of foreign matters such as dusts or fragments of molding material on said wide impinging face will hinder obtaining the proper stop position in the pressing operation. Particularly, after repeated pressing operations, there often results significant deterioration in the precision of stop position because of increase of such foreign matters on the impinging face, or damage on the impinging face by said foreign matters, leading to a shortened service life of the mold members.